For the uninitiated, Slender Man is a meme that originated on the Something Awful Forums. The Washington Post wrote a comprehensive guide about the creation of the fictional character:

On June 8, 2009, a new forum thread invited users [on Something Awful] to 'create paranormal images through Photoshop,' which many users did. But the creation of one user, Victor Surge, struck a particular chord: He posted two photos of children haunted by a tall, shadowy figure with tentacles for arms, along with blocks of ominous text:

we didn’t want to go, we didn’t want to kill them, but its persistent silence
and outstretched arms horrified and comforted us at the same time . . .
1983, photographer unknown, presumed dead.

For weeks, Surge continued posting doctored photos, newspaper clippings and child’s drawings of Slender Man, gradually pulling other users into the myth. They contributed their own Photoshops and stories, drawing parallels to older legends and nudging the story along.

But the fictional character quickly took on a life of its own. It left the boards of Something Awful and made its way across the internet. It became utterly divorced from its decidedly fictional origins, and in 2014, two 12-year-old girls in Wisconsin tried to murder their friend as a sacrifice to Slender Man. The victim survived after being brutally stabbed nineteen times. And, predictably, a Slender Man movie went into development at Sony in 2016.